1. Field of the Invention
This invention relates to the field of optical devices, and more specifically to the electrical detection and processing of coherent light.
2. Description of the Prior Art
The past decade and a half have seen the proposal of many approaches to the detection of coherent light in the presence of incoherent light. Some of these approaches have also rendered additional information about the detected source. To perform detection and other analysis in the presence of high levels of incoherent radiation, the detector must reject constant and fluctuating contributions due to incoherent light. Detection based upon both spatial and temporal coherence has been advocated.
Spatial coherence approaches have been suggested by J. Jannson, T. Jannson, and E. Wolf: "Spatial Coherence Discrimination In Scattering," Optics Letters, Vol. 13, No. 12, December 1988, pp. 1060-1062 and by U.S. Pat. No. 4,874,223 to O'Meara. The effectiveness of such approaches in detecting minute levels of coherent light obscured in incoherent light has not been demonstrated and does not exploit the large processing gain available to apparatus using time-integrating methods.
Many approaches have applied temporal modulation to the optical disturbance which preferentially operates on the coherent contribution. The signature of this modulation is then sought in detected optical intensity. Of these approaches, several produce modulation signatures which vary substantially with the incoming wavelength, so that searching of the signatures is required, making detection of strongly obscured coherent light impractical. Apparatus of this type is described by C. J. Duffy and D. Hickman: "A Temporal Coherence-Based Optical Sensor," Sensors and Actuators, Vol. 18, 1989, pp 17-31 and disclosed in U.S. Pat. Nos. 3,824,018 and 4,743,114 to Crane and U.S. Pat. No. 4,309,108 to Siebert.
The most promising methods for obtaining sensitivity while rejecting incoherent light are those which apply a periodic modulation. These produce intensity fluctuation components whose period is known precisely, allowing nearly arbitrary gain against the random fluctuations due to noise. Crane, in U.S. Pat. No. 4,735,507, discloses several such arrangements which effect the modulation by undesirable mechanical means. Crane's apparatus also permits determination of wavelength but in a manner which does not offer gain against noise and will not function when the coherent light is obscured by incoherent light.
Amodeo et al. in U.S. Pat. No. 4,595,292 and Krohn et al. in U.S. Pat. No. 4,600,307 have advocated the use of modulated Fabry-Perot etalons for modulation. Thick etalons--the form required here--are difficult to maintain in alignment. Amodeo et al. specifies the use of a liquid crystal in the etalon, severely limiting the modulation speed and thereby permitting only the monitoring of slow phenomena. Krohn et al. specifies that the etalon be modulated by the presence of an ultrasonic sound wave. Such a modulation restricts the optical aperture to less than the wavelength of the ultrasonic wave--a very small and undesirable value for practical systems. Neither Amodeo et al. nor Krohn et al. provides wavelength information.
U.S. Pat. No. 4,217,036 to Chang discloses apparatus which achieves modulation by periodic scanning of the pass band of an acoustooptic filter. Chang's apparatus provides a detected signal whose coherent component is impulsive, so that only a small fraction of the desired signal component is in the low harmonics of the periodicity. This results in great inefficiency and an attendant loss in processing gain. Moreover, spurious harmonics of the drive frequency may be produced by variations in the incoherent-light spectrum, even in the absence of coherent light. Chang does not offer a wavelength measurement.